1. Field of the Invention
The present invention relates to a negative-type resist composition which can be developed with a basic aqueous solution, and to a pattern-forming process. In recent years, greater integration of semiconductor integrated circuits has led to the realization of LSI and VLSI circuits, and the minimum line width of wiring patterns has reached the sub half-micron dimensions. This has required establishment of microfabricating techniques, and in the field of lithography this demand has been met with a solution whereby the wavelength of ultraviolet rays from the exposure light source is shifted toward the shorter wavelengths of the deep ultraviolet region, while much research has also been devoted to exposure methods employing light sources with wavelengths in the far ultraviolet region. At the same time, there has been accelerated development of resist materials which have lower light absorbance at shorter wavelengths and which have satisfactory sensitivity as well as high dry etching resistance.
2. Description of the Related Art
Recent years have seen more research into photolithography employing krypton fluoride excimer lasers (wavelength: 248 nm, hereunder abbreviated to KrF) as new exposure light sources for semiconductor fabrication, and these are beginning to come into practical use. Resists with high sensitivity and high resolution which can meet the short wavelength light sources include resist compositions based on the concept of "chemical amplification", which have been introduced by H. Ito et al. of IBM, U.S. (J. M. J. Frechet et al., Proc. Microcircuit Eng., 260 (1982), H. Ito et al., Digest of Technical Papers of 1982 Symposium on VLSI Technology, 86 (1983), H. Ito et al., "Polymers in Electronics", ACS Symposium Series 242, T. Davidson, ed., ACS, 11 (1984), U.S. Pat. No. 4,491,628 (1985)). The fundamental concept is based on utilizing a catalytic reaction in the resist film to improve the apparent quantum yield for higher sensitivity and resolution.
Referring to the example of a chemical amplification positive resist obtained by adding a PAG (Photo Acid Generator), which produces acid by light, to t-butoxycarbonylated (t-BOC) polyvinyl phenol (PVP), which has been very widely studied and utilized, post exposure baking (PEB) at the exposed sections of the resist causes the deprotection of the t-BOC protective groups, resulting in isobutene and carbon dioxide. The protonic acid generated in the deprotection reaction acts as a catalyst promoting a deprotecting chain reaction, thus resulting in considerable change in the polarity of the exposed regions. Accordingly, a resist pattern can be formed by selecting an appropriate developer.
Research has also been accelerating in recent years on lithography employing ArF (argon fluoride) excimer lasers at an even shorter wavelength (wavelength: 193 nm) for fabrication of devices with high monolayer integration, such as gigabit class DRAMs. At this wavelength the light absorption is stronger in conventional phenolic resins, and therefore the base resins must be changed. There has thus been an urgent need for development of resists which are suitable for such shorter wavelengths.
While much research has been conducted on conventional positive-type chemical amplification resists which are suitable for the ArF wavelength (for example, K. Nozaki et al., Chem. Mater., 6, 1492 (1994), K. Nakano et al., Proc. SPIE, 2195, 194 (1994), R. D. Allen et al., Proc. SPIE, 2438, 474 (1994), Japanese Unexamined Patent Publication No. 9-90637 and K. Nozaki et al., Jpn. J. Appl. Phys., 35, L528 (1996), K. Nozaki et al., J. Photopolym. Sci. Technol., 10(4), 545-550 (1997)), there have been few reports of monolayer negative-type chemical amplification resists, and even those have only been resists which include a crosslinker in the resist composition to obtain crosslinked negative patterns (for example, A. Katsuyama et al., Abstracted Papers of Third International Symposium on 193 nm Lithography, 51 (1997), Maeda et al., Extended Abstracts, the 58th Symposium on Applied Physics, No.2, 647(3a-SC-17) (1997)). Such negative resists take advantage of crosslinking reaction at the exposed region to increase the molecular weight, thus producing a difference in solubility in the developing solution with respect to the unexposed region, to obtain negative patterns. The issue of pattern swelling therefore is inevitable which limits to fabricate precise micropatterns.
Resolution enhancement techniques have been widely researched in recent years which employs phase shifting masks instead of binary masks, and this is a promising technique to obtain resolutions below exposure wavelengths. Negative-type resists are considered to be suitable when such masks are used, and thus ArF negative-type resists are also strongly desired from this standpoint as well. Such masks are considered applicable in cases which require resolution of 0.13 .mu.m or less with ArF light sources, and therefore efforts have been accelerating at developing resists which can resolve such micropatterns without swelling.